The present invention is in the field of plasma torches and in particular is directed to a plasma torch having an improved pilot arc generating circuit.
Plasma torches, otherwise known as electric arc torches, are known in the art for performing operations, such as cutting, welding, etc., on workpieces, and operate by directing a plasma consisting of ionized gas particles towards a workpiece. An example of the conventional single gas plasma torch is illustrated in Hatch, U.S. Pat. No. 3,813,510, assigned to the assignee herein. As the latter patent illustrates, a gas to be ionized, such as nitrogen, is fed through channels in the torch mechanism in such a manner as to swirl in front of the end of a negatively charged electrode. The welding tip which is adjacent the end of the electrode has a sufficiently high voltage applied thereto to cause a spark gap to jump between the electrode and the welding tip, thereby heating the gas and causing it to ionize. A pilot DC voltage between the electrode and the welding tip maintains the pilot arc. The ionized gas in the gap appears as a flame and extends externally of the tip where it can be seen by the operator. The extension of the pilot arc and the flame, which for practical purposes may be considered as being co-extensive, depends upon the power in the gap--i.e., the arc current--as well as the pressure of the gas forced into the gap and out of the torch. The pilot arc provides a source of light which enables the operator to see the proper position for the torch before starting the welding or cutting operation. In actual practice, when the pilot arc is on, a loop-shaped arc extending out of the torch can be seen. As the torch head is brought down towards the workpiece, the pilot arc jumps from the electrode to the workpiece due to the fact that the impedance of the workpiece current path is lower than the impedance of the welding tip current path.
Conventional single gas plasma torches include pilot arc circuits which provide a 20-40 amp. pilot arc current at 100-200 volts across the electrode-tip gap, resulting in an extension of the arc about 1/4-1/2 inch past the welding tip. As a consequence, the torch must be brought to within about 1/4-1/2 inch of the workpiece before the transfer arc jumps to the workpiece. This creates difficulties in the starting of cutting or welding operations.
Also, in the conventional plasma torch circuit, the pilot arc is initiated by a high-frequency signal which is generated in the power supply unit and travels the length of a relatively long cable to the plasma torch. This results in a dissipation of energy. Furthermore, the RF noise created by the conventional spark gap generating circuit can back up into other electrical systems connected to the torch, such as a numerical control machine for controlling patterned movement of the torch, and interfere with the operation of those machines.